It is more secure, but not that much more than using a single address if they are in the same wallet.If your wallet is compromised, you will lose all your keys anyways. It doesn't take more time to sweep the funds from multiple addresses.

What most people do is use a new address every time you make a new transaction.

If you truly want more security, you should create multiple wallets on different hardwares and split your funds across them.

Hi, do you think is a good security way spit my BTC in 20 different addresses?

1. If (and if...) a private key is cracked, I have still other 19/20 BTC2. I have a seed3. I have .dat wallet crypted backup

Is not it a good thing?

If someone steal my pc, i can ricreate the wallet fast and transfer all my BTC into an other pc...

Private keys are almost not possible to crack with the processor available at present , you may also find it impossible for Miners also and if you want to split your wallet into 20 address , there is no point of security ,hackers can still hack them as they can get into your wallet.getting into wallet is much easy than cracking private keys.

Hi, do you think is a good security way spit my BTC in 20 different addresses?

1. If (and if...) a private key is cracked, I have still other 19/20 BTC2. I have a seed3. I have .dat wallet crypted backup

Is not it a good thing?

If someone steal my pc, i can ricreate the wallet fast and transfer all my BTC into an other pc...

Private keys aren't crackable if the RNG is strong. The most probable scenario is your computer getting hacked. In that way, all your BTC would be gone too. If the wallet itself have weak RNG when generating transactions, you would likely realise it after spending one wallet and transfer your Bitcoins on other wallets away. If the seeds isn't random, then you would lose everything. I'll just ignore number 3 since it isn't useful if there's a seed.

This might be good idea, but those 21 addresses should not be in the same wallet. (If someone gets your wallet.dat and your password to it, he/she will access all those 21 addresses.) But you can split to different programs/machines (online wallets, smartphone wallets, paperwallet, Trezor, etc.)

It also depends on how you created these 21 addresses, for example if you created these online or at a computer which goes online then they are not safe, did you create them using electrum or other similar wallets because then someone could find all the addresses if they find the private key of one of those addresses.

So it is safe to split your coins in different addresses but you have to create these addresses securely and the safest way is doing it on an offline machine with a bitaddress.org source downloaded from github. Also, it is recommenced to have them encrypted, so even if someone gets hold of your paper wallet they won't be able to decrypt the key without the pass-phrase.

It also depends on how you created these 21 addresses, for example if you created these online or at a computer which goes online then they are not safe, did you create them using electrum or other similar wallets because then someone could find all the addresses if they find the private key of one of those addresses.

So it is safe to split your coins in different addresses but you have to create these addresses securely and the safest way is doing it on an offline machine with a bitaddress.org source downloaded from github. Also, it is recommenced to have them encrypted, so even if someone gets hold of your paper wallet they won't be able to decrypt the key without the pass-phrase.

Electrum and Multibit HD are the vulnerable ones. They generate all private keys from a seed so they need seeds to get all your private keys. Multibit Classic generates each key without link to one another.